1. Field of the Invention
The invention relates to a switching power converter and more particularly, to a switching power converter that applies a winding voltage sampler in the primary-side coil assembly of a transformer to regulate the on/off state of a switching device in the primary-side coil assembly based on deviation of the sampled voltage from a pre-set reference voltage.
2. Description of the Prior Art
Switching power converters have the advantages of high working efficiency and limited volume in size, and therefore, are widely used in a variety of electronic devices. FIG. 9 shows the circuit of a conventional flyback converter, including mainly an input circuit 1′, an output circuit 2′, a transformer T1′, and an optical coupler 3′.
Still referring to FIG. 9, the input circuit 1′ connects to an input voltage Vin and includes two main parts, a transistor Q1′ for switching, and, a controller 11′ for regulating PWM (Pulse Width Modulation). The transistor Q1′ connects to the primary-side coil assembly of the transformer T1′ at one end, and also connects to the output end of the controller 11′ with a PWM control at the other end. The input end FB of the controller 11′ connects to one end of the optical coupler 3′.
At the output end (indicated by a dotted rectangle on the right in FIG. 9) an output circuit 2′ connects to a secondary-side coil assembly of a transformer T1′. The output voltage Vout connects in parallel to one end of optical coupler 3′, making input circuit 1′ being segregated from output circuit 2′, and conveying output voltage Vout as a feedback to controller 11′ (of input circuit 1′). Consequently, controller 11′ is able to output a more stabilized voltage in correspondence to the feedback voltage by regulating the on/off states of transistor TI′.
To sum up, the above-mentioned prior art (illustrated in FIG. 9) makes use of the feedback control function of an optical coupler to manage stability of output voltages; the physical characteristics of an optical coupler unavoidably affects stability and durability of the system. For instance, the coupling efficiency of an optical coupler reflects the accuracy of an output voltage. Furthermore, extra electric elements are required to avoid or reduce the unstable performance of an optical coupler when it is used as a current-stabilizing current charger, adding more cost and bringing in undesired idleness and worn-out to the system.
FIG. 10 illustrates the electric circuits of another conventional flyback converter, including mainly an input circuit 5′ (indicated by a dotted rectangle on the left), a transformer T2′, an output circuit 6′ (as indicated by a dotted rectangle on the right). Unlike a conventional converter shown in FIG. 9, the transformer T2′ (shown in FIG. 12) includes three coils: two coils on the primary-side coil assembly (i.e. a primary-side 1st coil assembly, and a primary-side 2nd coil assembly) and one on the secondary-side coil assembly (i.e. a secondary-side 3rd coil assembly).
The input circuit 5′ connected to an input voltage Vin and includes two main parts: a transistor Q2′ for switching, and a controller 51′ for regulating PWM. One end of the transistor Q2′ connects to the primary-side 1st coil assembly N1′ (of the transformer T2′), and the other end, to the output end of the controller 51′. The input end of the controller 51′ connects to the primary-side 2nd coil assembly N2′ (of transformer T2′).
At the output end (indicated by a dotted rectangle on the right of FIG. 10), the output circuit 6′ connects to the secondary-side 3rd assembly N3′ (of transformer T2′) with an output voltage Vout. Deviations or variations in voltage conveyed from secondary-side coil assembly (of transistor T2′) to primary-side coil assembly (of transformer T2′) are detected by controller 51′. Controller 51′ regulates output voltages Vout by controlling the switching on/off of transistor Q2′ according to voltage deviations detected.
The above-mentioned conventional circuit has the merit of simplicity in structure, which, nevertheless, relies completely upon the physical characteristics of transformer T2′ for detecting changes in voltage and for regulating the circuit accordingly. Furthermore, the voltage conveyed back is not continuous in nature, affecting adversely system stability and durability, the no-load output voltage, and efficiency of dynamic voltage management.